Nonlinearity inherent to Power Amplifiers (PAs) used in radio communication systems may result in out-band spectrum spreading and in-band signal distortion of output signal power and further degrade the performance of the communication systems. At present, adaptive pre-distorters have been widely used in the wireless communication systems to compensate for the effects due to the nonlinearity of the power amplifiers.
An adaptive scalar pre-distorter samples out-band power of an output signal of a power amplifier and adjusts parameters of the pre-distorter adaptively to a change in power thereof to thereby achieve linearity of the power amplifier, as illustrated in FIG. 1. FIG. 1 illustrates a configuration of a power amplifying system 1 with an adaptive scalar pre-distorter. The power amplifying system 1 receives an input signal from a signal source 2 and provides the signal amplified in power to an antenna 3. The power amplifying system 1 includes a pre-distorting unit 14, a first Digital-Analog Converter (DAC) 15, an up-converter 16, a power amplifier 17, a feedback loop 18 and an algorithm updating module 19.
The pre-distorting unit 14 has nonlinearity corresponding to that of the power amplifier 17, and the signal pre-distorted by the pre-distorting unit 14 is processed by the first digital-analog converter 15 and the up-converter 16 and then amplified by the power amplifier 17 with nonlinear distortion into a resulting output signal in which nonlinear pre-distortion substantially cancels outs nonlinear distortion. Thus the signal output from the power amplifier 17 can be regarded as a result of linearly amplifying in power the input signal to the pre-distorting unit 14.
The feedback loop 18 obtains a power sample of the output signal of the power amplifier 17 and provides the algorithm updating module 19 with a cost function value for algorithm updating. The algorithm updating module 19 adjusts parameters of the pre-distorting unit 14 to the cost function value so that the nonlinearity of the pre-distorting unit 14 corresponds as closely as possible to that of the power amplifier 17. Upon convergence of the cost function value, it can be judged that the nonlinearity of the pre-distorting unit 14 has corresponded to that of the power amplifier 17.
As compared with an adaptive vector pre-distorter, this structure does not require synchronization for sampling points and thus lowers the complexity of implementing the feedback loop.
In the feedback loop 18 of FIG. 1, a dynamic range of an input signal to an Analog-Digital Converter (ADC) is typically adjusted through Automatic Gain Control (AGC) to lower the number of bit of the analog-digital converter. An automatic gain control algorithm module outputs a calculated gain value through the digital-analog converter to adjust a gain of a Variable Gain Amplifier (VGA). There is a requirement on the number of bits of the digital-analog converter in order for desired precision in adjusting the gain. As illustrated in FIG. 1, the feedback loop 18 includes a mixer 181, an Analog Band Pass Filter (ABPF) 182, a variable gain amplifier 183, an analog-digital converter 184, a cost function calculating module 185, an automatic gain control algorithm module 186 and a second digital-analog converter 187, where the automatic gain control algorithm module 186 provides the variable gain amplifier 183 with a variable gain g according to an analog-digital conversion result of the analog-digital converter 184.
A general idea of an existing automatic gain control algorithm is to ensure a specific statistical characteristic (e.g., average power, average amplitude, etc.) of an output signal of the analog-digital converter 184 to maintain a constant value all the time, as illustrated in FIG. 2. FIG. 2 illustrates a configuration of the feedback loop 18 in FIG. 1, where the automatic gain control algorithm module 186 includes a signal characteristic analyzing unit 186a and a gain generating unit 186b. Particularly for the automatic gain control algorithm to calculate in real time the statistical characteristic of the output signal of the analog-digital converter, the signal characteristic analyzing unit 186a needs to obtain an absolute value of the output signal of the analog-digital converter and compare the absolute value with a reference R, according to a result of which the gain generating unit 186b generates the variable gain g of the variable gain amplifier 183.
In this method, on one hand, the gain of the variable gain amplifier has to be adjusted in real time so as to ensure the statistical characteristic of the output signal of the analog-digital converter to be constant in real time, and on the other hand, the numbers of bits of the analog-digital converter and of the digital-analog converter in use may not be too low (for example, a 1-bit analog-digital converter may not be used).